Hi Chao-Jui,
You can write driver as part l4re and use block driver from l4linux. Or you can modify l4linux for direct access to hardware. We've done WiFi and USB-OTG support in l4linux for OMAP3 based board. But it was not easy. We've move a lot of platform specific code from arch/arm/mach-omap2 and arch/arm/plat-omap to l4 arch. Part of code required modification, for example, OMAP3 register mapping, restore startup sequence. We've chosen this way because USB-OTG and WiFi drivers are dificult for writing from scratch and this way allowed to keep our time.